New insights into growth and development of specific organ-tissue masses and energy flux can be acquired using recently developed imaging methods that probe previously unexplored aspects of human physiology. Young children have a high resting energy expenditure (REE) relative to their body weight and metabolically active compartment, fat-free mass (FFM). Both body weight and FFM are, however, metabolically heterogeneous and include organs and tissues varying widely in specific metabolic rate (i.e., organ REE/kg/d). One prevailing hypothesis is that most, if not all, of the higher REE observed in young animals and children compared to adults can be accounted for by a larger proportion of high metabolic rate components such as brain, liver, and heart. Until recently, FFM was the traditional and only means of adjusting REE for between-individual differences in metabolically active tissue components. Our laboratory has sought to improve the understanding of variation in REE by developing new and improved rapid magnetic resonance imaging (MRI) methods of quantifying all of the major heat producing organs and tissues in children and adults. Our long-term aim is to provide an improved understanding of human energy requirements. Specifically, we propose to test whether: 1) a portion of the elevated daily REE adjusted for FFM observed in young children (Tanner Stage 1) can be accounted for by the relative fractions of body mass as high metabolic activity tissues (HMT: heart, liver, kidney, brain) and low metabolic activity tissues (LMT; skeletal muscle, adipose tissue), 2) a portion of the age-related decline in daily REE adjusted for FFM observed in children can be accounted for by changes in the relative fractions of body mass as HMT and LMT during growth, and 3) REE differences may be explained by differences in growth patterns and mass of specific metabolically active compartments (liver, brain, kidneys, heart, skeletal muscle, adipose tissue) among lean and obese African-American, Asian and Caucasian children. This last aim explores in-detail the reported low REE observed in AA children and adults. Pilot studies carried out in our laboratory support variation in organ-tissue mass as the basis of race differences in REE. The proposed MRI data set to be acquired for testing our hypotheses and carrying out the specific aims will also be used to explore whether and how growth rates of the evaluated body compartments differ between lean and obese children. The proposed study will be carried out in 96 Tanner 1 girls and boys equally distributed by sex, race, and lean/obese, in a 2 X 2 X 3 factorial design (8 Ss/cell) at baseline, with two annual follow-ups in all subjects. The proposed studies will provide new insights into growth-related changes in REE and between-individual REE differences, notably those observed between race groups.